ARB: carbon intensity of biomethane from wastewater sludge could be as low as -65.27 g CO2e/MJ

22 May 2014

The staff of the California Air Resources Board (ARB) staff has posted three new Low Carbon Fuel Standard (LCFS) fuel pathway applications to the LCFS
public comments website: one for corn ethanol (from Heartland Corn Products in Minnesota) and one ARB staff-developed pathway (with two scenarios) for the production of biomethane from the mesophilic anaerobic digestion of wastewater sludge at a wastewater treatment plant (WWTP) located at a publicly-owned treatment works (POTW).

Under the LCFS, the baseline CI value for gasoline was 95.86 g CO2e/MJ; for diesel fuel, 94.71 g CO2e/MJ. Staff estimated the carbon intensities (CIs) for biomethane produced under two alternative scenarios; under the first scenario, the CI of biomethane is 10.86 g CO2e/MJ; under the second, the CI is -65.27 g CO2e/MJ—i.e., it generates a credit.

The first scenario is for biomethane produced at a Small-to-Medium POTW with wastewater inflows of 5-20 million gallons per day (MGD). In this model, only a small parasitic load on the biogas produced is used to heat the digesters. Grid-based electricity, using the California-Marginal mix of electrical generating assets, is assumed to power the wastewater sludge treatment, and biogas cleaning, compression, and fuel dispensing processes.

The second scenario is for a Medium-to-Large POTW with wastewater inflows of 21-100 MGD. In this model, the majority of the biogas is allocated to the production of renewable power using a device such as a gas-fired turbine with an exhaust heat recovery system. The balance of the biogas produced in the digesters is allocated to on-site vehicle fueling, or compression and distribution through the natural gas grid for purposes of off-site vehicle fueling.

Heat recovered from the exhaust of the combustion gases produced by power generation sustains the mesophilic thermal requirements of the anaerobic digesters. The electricity required for the wastewater sludge treatment and biogas cleaning, compression, and dispensing processes is provided by the renewable power generated on-site by the compliant device.

This scenario also predicts that surplus electrical power will be generated, and that this power will be exported, displacing California Marginal electricity on the electrical grid. Therefore, this model accrues an additional LCFS credit for lowering the GHG impacts of grid-based California Marginal electrical generation.

Both models include a credit for avoided flaring emissions. Staff assumes that due to regulatory and air quality non-attainment considerations, flaring of the biogas to achieve near complete destruction of the volatile components in the biogas with high global warming potentials is the only available option for the reference case.

Any productive use of the biogas, such as for vehicle fuel or for the production of renewable electrical power, avoids the emissions and energy loss caused by flaring of the biogas.

Background. Wastewater sludge is generated from the primary and secondary treatment processes designed for the municipal wastewater that flows into the WWTP. Since the wastewater sludge content is primarily organic material, California State and local laws require further treatment of the wastewater sludge prior to discharge or disposal. One of the most common processes for the treatment of wastewater sludge at a POTW is the anaerobic digestion of the sludge under mesophilic operating conditions (~35 ˚C).

Anaerobically digesting the wastewater sludge destroys part of the organic matter and produces biogas, a mixture comprised of methane (CH4) and carbon dioxide (CO2), along with some trace impurities such as hydrogen sulfide, siloxanes, and vinyl chloride. Since both major components of biogas are greenhouse gases (GHG), the biogas produced is further destroyed by flaring (methane capture and destruction), or used in a device that generates electricity from the combustion of the biogas.

An alternative fate for the biogas, which is contains ~58 vol% methane, is to further refine the biogas to remove the carbon dioxide and other trace impurities to produce near-pure biomethane (>99% CH4). This biomethane can be compressed and sold as a vehicle fuel either on-site, or injected into the common carrier pipeline for fueling at an off-site location.